Lasers are ubiquitous; they are used for many different purposes and in a variety of applications. One type of semiconductor laser that has recently garnered much attention is the vertical cavity surface emitting laser, commonly known by the acronym “VCSEL.”
Semiconductor diode-lasers comprise a plurality of layers of semiconductor material that is sandwiched between two “mirrors.” Photon generation occurs principally in an optical gain region, where charge carriers (i.e., electrons and holes) from n- and p-doped regions of the device combine and emit energy as photons. The mirrors force most of these emitted photons to return through the gain region. As they do, the photons stimulate the recombination of other charge carriers, which in-turn generates more photons. These newly generated photons are characterized by the same wavelength as the photons that stimulated their emission.
The mirrors and gain region collectively define a “laser cavity.” Light is emitted from the laser cavity by virtue of the fact that the mirrors are not one-hundred percent reflective, which enables some photons to escape the laser cavity as emitted light. In the more conventional “edge-emitter,” the laser cavity is in the plane of the junction layer and light is emitted parallel to the substrate surface. In contrast, the mirrors that define the laser cavity in a VCSEL are disposed above and below the junction layer. As a consequence, the VCSEL comprises a “vertical cavity,” and light is emitted in a direction that is perpendicular to the substrate surface.
By virtue of its geometry, the VCSEL offers numerous performance, manufacturability, and cost advantages over conventional edge-emitting lasers in some applications. Advantages include:                A single VCSEL is the smallest commercially available semiconductor laser diode type. Each individual laser fits on a chip area smaller than 0.25 to 0.25 mm. And most of that area is used for electrical contacts and marking. The intrinsic diameter of the active laser is smaller then that of a human hair. This enables mass market production similar to the technology used in the silicon industry.        The structure can easily be monolithically integrated in one- and two-dimensional array configurations.        Low threshold currents enable high-density arrays (Terabit aggregate communication).        
Surface-normal emission and a geometry that is nearly identical to a photo detector results in simplified alignment and packaging.                Low-cost potential because the devices are completed and tested at the wafer level.        Circular and low divergence output beams eliminate the need for corrective optics.        Lower temperature-sensitivity compared to edge-emitting laser diodes.        High transmission speed with low power consumption.        
VCSELs provide enhanced performance benefits for many applications, such as local area networks (LAN), optical wireless, telecommunication switches, optical storage, sensing applications, gas detection, absorption spectroscopy, night vision, homeland security applications, military applications (e.g., range finding and target localization, etc.), medical applications (e.g., low-level laser therapy, etc.), and automotive data communications, among others. In fact, VCSELs are the light emitter of choice for high-performance fiber data communications (speeds much higher than 1 Gbs) like Gigabit Ethernet.
Notwithstanding its many advantages over other types of coherent light sources, VCSELs do possess characteristics that, for some applications, are disadvantageous. In particular, relative to other lasers, most VCSELs have a relatively broad spectral bandwidth (i.e., about 0.3 nanometers). For some applications, this “broad” spectral bandwidth is unacceptable or otherwise undesirable.
To narrow the spectral bandwidth of a VCSEL, it is known to couple an external mirror, such as a Bragg grating, to a VCSEL. The external-cavity Bragg grating is capable of reducing the spectral bandwidth of the VCSEL to <<0.1 nanometers. But a Bragg grating is relatively expensive to implement and suffers reliability issues (to the extent that the grating is stretched for the purpose of tuning).
U.S. Pat. No. 6,690,687 addresses a similar issue; how to tune (i.e., narrow) the spectral bandwidth of a widely tunable semiconductor laser, such as for use in a dense wavelength division multiplexing (“DWDM”) system. This patent discloses a widely tunable semiconductor laser having a cavity with a ring resonator, a Mach-Zehnder interferometer, and tuning elements for both the ring and interferometer. The laser is formed on a III/V material semiconductor “gain” chip. The ring resonator and Mach-Zehnder interferometer reside on a silicon semiconductor “tuning” chip.
The gain chip generates light within about 10 nm of a selected channel (e.g., WDM channel, etc.). In the absence of the tuning chip, the wavelength of the light can vary throughout the tuning range as a function of the applied current. To prevent this variation of wavelength, the ring resonator and Mach-Zehnder tuning port are used.
The ring resonator operates as a fine-tuning device under the control of a fine-tuning control unit (e.g., a heater, etc.) to limit resonance within the laser cavity to a set of sharp resonance peaks within the gain range of the laser. The Mach-Zehnder interferometer operates as a wide-tuning port under the control of a wide-tuning control unit (e.g., a heater, etc.) to limit the resonance within the laser cavity to a profile having a single broad peak. The Mach-Zehnder accomplishes this by effectively selecting one of the plural peaks generated by the ring resonator and suppressing the amplitude of the other peaks. According to the patent, the result is that the laser cavity resonates primarily at the selected emission wavelength and transmission sidebands are substantially reduced.
Although a VCSEL is a semiconductor laser, U.S. Pat. No. 6,690,687 does not contemplate the use of a VCSEL. In this regard, VCSELs do not have a sufficiently broad spectral bandwidth for that application.
Furthermore, there are some formidable problems that would arise if a VCSEL were to be used in such a system. In particular, the issue of how to efficiently launch light that is emitted from a VCSEL into a waveguide must be addressed. The problem here is that although the light emitted from a VCSEL is orthogonal to the substrate, the waveguides for transmitting the light are in the plane (parallel) to the substrate. Launching light from an edge-emitter is far less problematic; it is simply an issue of vertical alignment.
Another problematic issue with VCSELS pertains to their operating wavelength. Most material systems that are useful for waveguides are not transparent at 850 nanometers, which is a common operating wavelength of VCSELs. And most material systems that are useful as waveguides at 850 nanometers have relatively high attenuation.
Inapplicability to VCSELs aside, the arrangement for narrowing the spectral bandwidth of a widely-tunable laser source that is disclosed in U.S. Pat. No. 6,690,687 has a variety of drawbacks.
In particular, the approach adopted in U.S. Pat. No. 6,690,687 is complex, both in terms of components and layout. As to components, that arrangement requires a Mach-Zehnder interferometer and a means to control it, as well as a ring resonator and means to control it. As to layout, the arrangement disclosed in U.S. Pat. No. 6,690,687 is a hybrid system; that is, the laser and “tuning” devices (ring resonator and Mach Zehnder) reside on different chips. This complication has both cost and performance implications.
In particular, as compared to a single chip and/or monolithic system, the hybrid system is substantially more expensive due to labor and materials costs, and problematic due to the necessity for multiple suppliers, not to mention the need to meet various opto-mechanical packaging requirements. Regarding performance, the use of different materials systems—a III/V chip and a silicon chip—means that there will be differences in the thermal expansion of the two chips. This causes alignment problems, which are dealt with by either incurring the expense of special packaging or simply operating with the errors.
The benefits of monolithic integration are well known, but in the context of U.S. Pat. No. 6,690,687, such integration would be very problematic. Among other any other issues, the III/V wafer (on which the laser is grown) is very fragile. If an attempt were made to grow the silicon dioxide/silicon oxynitride waveguide disclosed in U.S. Pat. No. 6,690,687 on a III/V wafer, the stresses that develop in the (relatively thick) waveguides that are being formed would impart strain in the wafer that is almost certain to fracture it. Also, unless the semiconductor is an edge emitter, the previously-mentioned difficultly of coupling light from a VCSEL into a planar waveguide must be addressed.
A need therefore exists for a relatively less complex approach to creating a narrow-spectrum semiconductor laser source, wherein the semiconductor laser can be a VCSEL.